More than three decades have passed since Catalina Aguado and her husband discovered a monarch butterfly sanctuary in a Mexican forest — and helped solve the mystery of the monarch's migration.

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A young boy in Canada wondered where butterflies go in the winter — and spent 40 years trying to answer that question.

In 1973, Dr. Fred Urquhart — all grown up by then — placed an ad in a newspaper in Mexico looking for volunteers to tag and observe butterflies and find their destination. A woman named Catalina Aguado and her American husband, Kenneth Brugger, answered that ad. They spent two years searching in remote parts of Mexico.

"We were free-spirited, both of us, and we loved the adventure, so we never felt discouraged," Aguado said. "We had a jeep, we used a motor home, we went on horses. You know, a very difficult task, but we loved the idea."

Finally, she and her husband came upon a sanctuary in the forest, where they found of hundreds of millions of butterflies.

"I just called at him, 'I see them! I see them!' And of course, after that, we went silent," Aguado remembers.

Now, more than 35 years later, documentarian Mike Slee has written, produced and directed Flight of the Butterflies, a 3-D IMAX film about the migration of the monarch to sanctuaries like the one Aguado and Brugger came upon in Mexico.

"What you see, you can't imagine nature ever being like this," Slee says. "Trees that are draped — that are made, almost, of butterflies. It's got a surreal, supernatural feeling to it. It sends a sort of tingle up your spine when you see it in 3-D. And then they wake up and they all begin to fly."

The migration of the monarch butterfly is a staggering natural phenomenon. It takes two or three generations for the creatures to make their way north to Canada — but then one "supergeneration" makes the 2,000-mile trip back to Mexico for the winter.

And distance isn't the only thing the delicate creatures have to contend with. Inclement weather, deforestation and human threats like plows and crop-dusters also speckle their route southward.

The filmmakers faced challenges of their own. Slee and his team considered balloons, helicopters and cables for filming. Ultimately, they used a 70-foot crane to get up close.

"We can see the scales on the wings of the butterfly," he says. "We can see the punk hairstyle on the butterfly. And it's — for the scientists it's been amazing watching their reaction, because they've never been able to look so closely at the living animal."

Aguado was transported by the experience of revisiting her quest in 3-D. She says this new film is able to communicate what her words never could.

"After, what, 36 years? I can say wonderful, fantastic and glorious — and whatever other words, but I cannot describe the feeling. It was magical," Aguado says.

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Transcript

RENEE MONTAGNE, HOST:

Decades ago, a boy in Canada wondered just where butterflies go in the winter. Once he grew up, he chased after them and his effort is now the subject of a new IMAX film that opened this week.

STEVE INSKEEP, HOST:

Dr. Fred Urquhart spent almost 40 years tracking butterflies as they flew south and back north again with the seasons. He started in 1973, placing an ad in Mexican newspapers seeking volunteers to tag and observe butterflies as they flew south. A woman named Catalina Aguado and her American husband, Kenneth Brugger, answered that ad.

CATALINA AGUADO: We were free-spirited, both of us, and we loved the adventure, so we never felt discouraged.

MONTAGNE: They spent two years searching remote parts of Mexico. They went on foot, on motorcycle.

AGUADO: We had a jeep, we used a motor home, we went on horses. You know, a very difficult task, but we loved the idea.

INSKEEP: Finally, Aguado and her husband came upon a forest with hundreds of millions of butterflies.

AGUADO: I just called at him: I see them, I see them. And of course after that we went silent.

MIKE SLEE: What you see you can't imagine nature ever being like this.

INSKEEP: That's Mike Slee, who co-wrote, produced and directed a 3-D IMAX film about the discovery.

SLEE: Trees that are draped - that are made, almost - of butterflies. It's got a surreal, supernatural feeling to it. It sends a sort of tingle up your spine when you see it in 3-D. And then they wake up and they all begin to fly.

MONTAGNE: It takes two to three generations of the butterfly - each living several weeks - to go north to Canada and one super-generation - living six to eight months - to migrate 2,000 miles back to Mexico for the winter, all the while contending with weather, deforestation and human threats like farm machinery and crop-dusters.

INSKEEP: An incredible effort by the butterflies and not so easy for the filmmakers either. Mike Slee and his team considered balloons, helicopters, cables and wires for filming, and ultimately used a 70-foot crane to lift the cameras up close to the butterflies.

SLEE: We can see the scales on the wings of the butterfly. We can see the punk hairstyle on the butterfly. And it's - for the scientists it's been amazing watching their reaction, because they've never been able to look so closely at the living animal.

MONTAGNE: Catalina Aguado was transported by revisiting her experience in 3-D.

AGUADO: After - what, 36 years? - I can say wonderful, fantastic and glorious and whatever other words, but I cannot describe the feeling. It was magical.

INSKEEP: For an excerpt from the film "The Flight of the Butterflies" in 3-D, go to NPR.org. This is NPR News. Transcript provided by NPR, Copyright NPR.